In recent years, the design rule for LSI has been progressively adopting more micronized orders of sub-quarter micron, and as a result the shortening of the wavelength of the exposure light source has been compelled. Thus, the exposure light source has been shifted from g-line (436 nm) and i-line (365 nm) lights of the heretofore commonly used mercury lamp to KrF excimer lasers (248 nm) and ArF excimer lasers (193 nm).
In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display board, a pattern is made by irradiating a light to a semiconductor wafer or an original plate for liquid crystal; however, if a dust adheres to a mask for lithography (also simply referred to as “mask”) or a reticle (these are collectively referred to as “exposure original plate” herein below) used in this pattern creating stage, the dust absorbs light or bends it, causing deformation of a transferred pattern, roughened edges or black stains on a base, and leads to problems of damaged dimensions, poor quality, deformed appearance and the like. Now, we will explain this by taking a mask for example.
The above-mentioned lithography works are usually performed in a clean room, but even so, it is still difficult to keep the exposure original plate clean all the time; therefore, in general the light irradiation is carried out only after a surface of the exposure original plate is sheltered by a pellicle. In this way, the dust particle is prevented from directly adhering to the surface of the exposure original plate but is caught on the pellicle, and if, at the time of the lithography, the exposure light is focused on the pattern described on the exposure original plate the dust particle on the pellicle does not partake in the image transferring.
In general, a pellicle is mainly built up of a pellicle frame, a pellicle membrane and an agglutinant layer; and the pellicle membrane is attached to an annular face of the pellicle frame via an adhesive in a slack-free manner, and the agglutinant layer for agglutinating the pellicle to an exposure original plate is formed on another annular face of the pellicle frame. The pellicle membrane is made of cellulose nitrate, cellulose acetate and a fluorine-containing polymer or the like which transmit well such lights that are used in light exposure (e.g., g-line [430 nm], i-line [365 nm], KrF excimer laser [248 nm], and ArF excimer laser [193 nm]).
To attach the pellicle membrane to the pellicle frame, a solvent that dissolves the pellicle membrane well is applied to one of the two annular faces (hereinafter referred to as “upper annular face”) and, after pasting the pellicle membrane on it, the solvent and the membrane are dried by air flow, or alternatively an adhesive agent such as acrylic resin, epoxy resin and fluorine-containing resin is used to fix the pellicle membrane on the upper annular face of the pellicle frame. On the other hand, the other one of the two annular faces (hereinafter referred to as “lower annular face”) is laid with the agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin and a silicone resin or the like for agglutinating the pellicle frame to the exposure original plate, and on this agglutinant layer is laid a releasable liner for protecting the agglutinant layer if it is required.
The pellicle as described above is installed for the purpose of preventing the dust from adhering to the exposure original plate, and the installation is effected as the agglutinant layer of the pellicle is pressed upon the exposure original plate, whereby a closed space defined by the pellicle membrane, the pellicle frame and the exposure original plate is created in a manner such that the pellicle frame entirely surrounds the pattern region formed in the surface of the exposure original plate. Thus, the pattern region is isolated from the external atmosphere by means of the pellicle so that the dust outside the pellicle cannot reach the pattern region.
However, although such a protective pellicle can prevent entrance of any dust particle after the pellicle is installed, there is a possibility that some dust particles are already adhering to the pellicle membrane or the inner wall of the pellicle frame as of the time of pellicle installation. It is normally possible to detect a dust particle on the pellicle membrane if its size is no smaller than about 0.3 micrometer, so that most of the dust particles can be removed from the pellicle membrane through detection; however, with regard to the dust particle adhering to the inner wall of the pellicle frame, it is not possible to detect it unless it is 10 micrometers or larger, so that no small number of dust particles having a size of less than 10 micrometers may exist on the inner wall of the frame, which have escaped the detection, and these undetected dust particles may fall on the exposure original plate during the use of the pellicle, and cause problems.
IP Publication 1 discloses a technology to solve this kind of problem, according to which the frame of the pellicle is coated with an adhesive such as epoxy resin whereby the undetectable dust particles adhering to the pellicle frame are stuck and immobilized by the adhesive lest they should fall onto the pattern face.    [Prior Art]    [IP Publication]    [IP Publication 1]
Japanese Pre-Grant Publication S61(1986)-241756